1. Field of the Invention
The present invention relates to a technique of recognizing the type of an optical disc that has been loaded into an optical disc drive, which is compatible with multiple types of optical discs including compact discs (CDs), digital versatile discs (DVDs) and Blu-ray discs (BDs). The present invention also relates to a technique of focusing a light beam on a desired one of multiple information storage layers of an optical disc and optimizing the spherical aberration thereof.
2. Description of the Related Art
In reading data from an optical disc, the conventional optical disc drive irradiates the optical disc with relatively weak light having a constant quantity and detects the light that has been reflected from the optical disc. For example, the optical disc drive disclosed in Japanese Laid-Open Publication No. 52-80802 irradiates an optical disc, including pits and spaces with mutually different physical properties, with light. In this case, when reflected from the optical disc, the light has its intensity modulated by those pits and spaces. Thus, the data can be read from the optical disc by way of the reflected light. On the other hand, in writing data on the optical disc, the optical disc drive adaptively changes the quantity of light according to the data to be written and then irradiates a material film (i.e., an information storage layer) on the optical disc with such light in variable quantities. As a result, marks and spaces, representing the data, are formed on the optical disc.
On a read-only optical disc, pits and spaces are arranged spirally. On the other hand, on an optical disc from/on which data can be read and written, an information storage layer is deposited by an evaporation process, for example, over spiral grooves and tracks (i.e., areas between the grooves). In this case, data is read from, and written onto, the information storage layer by an optical technique.
To read or write information from/on an optical disc, a focus control and a tracking control are needed. The “focus control” means controlling a focal point, that is, a beam spot of a light beam perpendicularly to the surface of a given optical disc (which direction will be referred to herein as a “focusing direction”) such that the light beam can always maintain a predetermined focusing state on the recording material film. To control the beam spot of a light beam with respect to a predetermined information storage layer will be referred to herein as a “focus control on a predetermined information storage layer”. On the other hand, the “tracking control” means controlling the beam spot of a light beam along the radius of a given optical disc (which direction will be referred to herein as a “tracking direction”) such that the beam spot is always located right on a target track.
Hereinafter, a conventional optical disc drive will be described with reference to FIG. 14, which shows an arrangement of functional blocks for a conventional optical disc drive 510. As shown in FIG. 14, the optical disc drive 510 includes a semiconductor laser diode 11 and an objective lens 13 (which together function as emitting/focusing means), a focus actuator 14 (which functions as focus moving means), an FE generator 20 (which functions as focus error detecting means) and a focus drive signal generator 22 (which functions as a focus driving means or a focus driver).
More specifically, an optical head 10 is made up of the semiconductor laser diode 11, the objective lens 13, a beam splitter 12, the focus actuator 14, and a photodetector 15. A light beam, which has been emitted from the semiconductor laser diode 11, is passed through the beam splitter 12 and then focused onto the optical disc 1 through the objective lens 13. After having been reflected from the optical disc 1, the light beam is passed through the objective lens 13 again, reflected from the beam splitter 12 and then incident onto the photodetector 15. The objective lens 13 is supported by an elastic body (not shown). When a predetermined amount of current flows through the focus actuator 14, the objective lens 13 is moved in the focusing direction due to an electromagnetic force. In this manner, the beam spot of the light beam can be changed. On detecting the incident light beam, the photodetector 15 outputs a signal, representing the detected light, to the FE generator 20.
Based on the signal supplied from the photodetector 15, the FE generator 20 obtains a signal representing the focusing state of the light beam with respect to the information storage layer of the optical disc 1 (i.e., a focus error signal representing the deviation of the beam spot of the light beam from the information storage layer of the optical disc 1) and outputs the focus error signal to a disc type recognizer 43. The focus drive signal generator 22 generates a drive signal and outputs it to the focus actuator 14. In response to the drive signal, the focus actuator 14 moves the objective lens 13 either toward, or away from, the optical disc 1. This drive signal is also supplied to the disc type recognizer 43.
Next, it will be described with reference to FIG. 15 how this optical disc drive 510 performs a disc type recognition operation. In FIG. 15, portion (a) shows the waveform of the output signal of the focus drive signal generator 22, while portion (b) shows the waveform of the output signal of the FE generator 20. In FIG. 15, the abscissa represents the time and the ordinate represents the signal level.
The focus drive signal generator 22 starts moving the beam spot of the light beam from a position, which is sufficiently away from the optical disc 1, toward the optical disc 1. When the beam spot of the light beam reaches the information storage layer of the optical disc 1, the output signal of the FE generator 20 has a zero level as indicated by the legend “location of information storage layer” in FIG. 15B. This is a so-called “zero cross state”. When the output signal of the FE generator 20 is in the zero cross state, the disc type recognizer 43 detects the depth of the information storage layer of the optical disc 1 based on the level of the output signal of the focus drive signal generator 22. In this case, the distance from the home position of the focus actuator 14 to the surface of the optical disc 1 is constant, and the distance for which the focus actuator 14 can move in response to the output signal of the focus drive signal generator 22 is also constant. Thus, the depth of the information storage layer of the optical disc 1 can be detected easily. The depth of the information storage layer of one type of optical disc as measured from the surface thereof is different from that of the information storage layer of another type of optical disc. Accordingly, the optical disc drive 510 can recognize the type of the given optical disc 1 according to the depth detected.
Next, another conventional optical disc drive will be described with reference to FIG. 16, which shows an arrangement of functional blocks for the conventional optical disc drive 520. When an optical disc 1 including multiple information storage layers is loaded into this optical disc drive 520, the optical disc drive 520 performs a focus control on a desired one of the information storage layers of the optical disc 1 such that the beam spot of the light beam is located right on the desired information storage layer. In FIG. 16, each component of the optical disc drive 520, having substantially the same function as the counterpart of the optical disc drive 510 shown in FIG. 14, is identified by the same reference numeral and the description thereof will be omitted herein.
In this optical disc drive 520, the output signal of the FE generator 20 is supplied to a focus filter 21 and a selector 45. In response to the output signal of the FE generator 20, the focus filter 21 compensates for the phase for the purpose of a focus control. Then, the focus filter 21 outputs the phase-compensated signal to the selector 45. The selector 45 selects either the output signal of the focus drive signal generator 22 or the output signal of the focus filter 21 and then supplies the selected signal to the focus actuator 14.
Next, it will be described how the optical disc drive 520 performs a focus control process. While the optical disc drive 520 is performing no focus control (i.e., while the focus control is OFF), the selector 45 selects the output signal of the focus drive signal generator 22 and then passes it to the focus actuator 14. In response, the focus actuator 14 brings the beam spot of the light beam from a position that is sufficiently distant from the optical disc 1 closer to the optical disc 1. When the beam spot of the light beam reaches the information storage layer of the optical disc 1, the output signal of the FE generator 20 has a zero level as indicated by the legend “location of information storage layer” in FIG. 15B. At this zero-cross timing of the output signal of the FE generator 20, the selector 45 switches the signals to be output selectively to start passing the output signal of the focus filter 21 to the focus actuator 14. As a result, the beam spot of the light beam is controlled with respect to the information storage layer of the optical disc 1. In this case, the information storage layer to be subjected to the focus control is closest to the surface of the optical disc on which the light beam is incident. Conversely, if the focus actuator 14 separates the beam spot of the light beam away from the position that is sufficiently close to the optical disc 1, then the information storage layer to be subjected to the focus control is most distant from the surface of the optical disc 1 on which the light beam is incident. In this case, the focus control on that information storage layer is also started as described above.
Next, still another conventional optical disc drive will be described with reference to FIG. 17, which shows an arrangement of functional blocks for the conventional optical disc drive 530. The optical disc drive 530 can regulate the spherical aberration according to which information storage layer the light beam should be focused on. For that purpose, the optical disc drive 530 includes an aberration generator 16 and an aberration setter 30 as a spherical aberration generating means and an aberration regulator 32 as a spherical aberration regulating means. In FIG. 17, each component of the optical disc drive 530, having substantially the same function as the counterpart of the optical disc drive 510 or 520 shown in FIG. 14 or 16, is identified by the same reference numeral and the description thereof will be omitted herein.
In this optical disc drive 530, the output signal of the photodetector 15 is supplied to the FE generator 20 and a TE generator 24. The focus error signal, i.e., output signal of the FE generator 20, is supplied to the focus filter 21, subjected to predetermined processing there, and then passed to the focus actuator 14. Meanwhile, in accordance with the output signal of the photodetector 15, the TE generator 24 generates a tracking error signal TE, representing a positional relationship between the beam spot and a target track on the optical disc 1, and supplies it to an amplitude detector 25. In response, the amplitude detector 25 detects the amplitude of the tracking error signal TE that has been supplied from the TE generator 24 and passes the result to the aberration regulator 32.
The aberration regulator 32 not only stores the signal supplied from the amplitude detector 25 but also generates a setting signal from it and supplies the setting signal to the aberration setter 30. On receiving the output signal of the aberration regulator 32, the aberration setter 30 outputs an aberration setting signal, which defines the spherical aberration to be generated at the beam spot of the light beam, to the aberration generator 16. In accordance with the aberration setting signal, the aberration generator 16 changes the spherical aberration of the light beam. As a result, the tracking error signal generated by the TE generator 24 is changed and output to the amplitude detector 25 again. This feedback process is carried out so as to change the spherical aberration within a predetermined range. The aberration regulator 32 draws up and stores thereon a table of correspondence showing the relationship between the output signal of the amplitude detector 25 and the magnitude of spherical aberration generated. Then, the aberration regulator 32 finds a setting signal, which defines a spherical aberration that is associated with the output signal of the amplitude detector 25 with the maximum signal level, and outputs that setting signal to the aberration setter 30.
Next, it will be described with reference to FIG. 18 how the optical disc drive 530 regulates the spherical aberration. In FIG. 18, portion (a) shows the waveform of the setting signal being output from the aberration regulator 32, while portion (b) shows the waveform of the output signal of the amplitude detector 25. In FIG. 18, the abscissa represents the time and the ordinate represents the signal level.
As shown in portion (a) of FIG. 18, the aberration regulator 32 outputs a ramp signal with a predetermined amplitude to the aberration setter 30 before the time t0. The tracking error signal to be generated in accordance with such a setting signal has the maximum amplitude when the spherical aberration at the beam spot of the light beam is minimized. However, as the spherical aberration increases, the amplitude of the tracking error signal decreases. Accordingly, a setting signal that is associated with the output signal of the amplitude detector 25 with the maximum level as shown in portion (b) of FIG. 18 should define the best spherical aberration. Thus, the aberration regulator 32 stores the level of the setting signal that is associated with the output signal of the amplitude detector 25 with the maximum level, and starts outputting the stored setting signal to the aberration setter 30 at the time t0. As a result, the tracking error signal will maintain the maximum amplitude and the output signal of the amplitude detector 25 will also keep the maximum amplitude from the time t0 on as shown in portion (b) of FIG. 18. In this manner, the spherical aberration at the beam spot of the light beam can be regulated to the best value.
The conventional optical disc drives 510, 520 and 530, however, have the following drawbacks.
First, in the conventional optical disc drive 510 shown in FIG. 14, the depth of the information storage layer of the given optical disc 1 may not be detected so accurately due to possible variations in the sensitivity of the focus actuator 14 or other circuits. Also, since the optical disc 1 should flutter due to its own rotation and due to vibrations caused by the driving mechanism of the optical disc drive, the information storage layer is constantly movable vertically, thus potentially causing errors in the detected depth of the information storage layer. If such an error is a serious one, then the type of the loaded disc may be recognized erroneously.
On the other hand, if the optical disc 1 to be loaded has three or more information storage layers, the conventional optical disc drive 520 shown in FIG. 16 cannot perform any focus control on the intermediate information storage layer(s). Also, if the optical disc drive 520 attempts to perform a focus control on the information storage layer that is most distant from the surface of the optical disc 1 on which the light beam is incident, the objective lens 13 may happen to contact with the optical disc 1. This is because the beam spot of the light beam needs to be initially located deeper than the information storage layer at the beginning of a focus control. Thus, while the focus control is being carried out on that deep information storage layer, the objective lens 13 is too much close to the optical disc 1.
Furthermore, in the conventional optical disc drive 530 shown in FIG. 17, the focus error signal generated by the FE generator 20, as well as the tracking error signal generated by the TE generator 24, may decrease its amplitude as the spherical aberration increases. Thus, the focus control cannot be carried out stably. This is because the spherical aberration at the beam spot of the light beam is still far from its best value for a while after the focus control operation has been started. Then, even if a tracking control operation is started in order to generate the tracking error signal, it is difficult to measure the amplitude of the tracking error signal. As a result, it is also hard to regulate the spherical aberration to its best value.